Hello !
Sorry to be so late in my answer, I was not on the forum during some days.
The trip you plan is long and there are hundreds of nice places to visit between Saint Malo and Calais. In my opinion, somebody who would see just 10% needs two or three months...
My feeling is, but it's just my feeling, it's better to well visit a small area than to have a so long travel which makes you lack many places because it's impossible to see all in one time, especially in 3 days.
This being said, Saint Malo merits one or two days. It's a lovely small city with beautiful houses surrounded by strong fortifications.
Here is a link to know more about Saint Malo: http://www.saint-malo-tourisme.co.uk/
Mont Saint Michel, at about 40 km East of Saint Malo, is a major place in this area. It's one of the best historical place of France, but also one of the most submerged by tourists.
Another link: http://www.ot-montsaintmichel.com/index.htm?lang=en
An agreable trip can be done along the west coast of Manche department, from Granville to Cherbourg. Beautiful and calm beachs, many small fishing harbours.
http://www.manche-tourism.com/en
Continuing to East, you will see the landings beachs on June 1944 (Utah beach, Omaha beach, Gold beach, Juno beach, Sword beach.......) but also some nice cities as for instance Port en Bessin (photo below).

IMAGE LINK: http://www.hostingpics.net ….php?id=378950Orange2.jpg
Inland, at 20 or 30 km from the coast you will find Bayeux and Caen with their historical great monuments.
http://bayeux-bessin-tourisme.com/en/
and
http://www.caen-tourisme.fr/en
More at East, and after crossing river Seine by the majestic "pont de Normandie" you will arrive in one of the ugliest city of France: Le Havre. (it's just my opinion

)
But, after this charmless heap of concret you will ride your scooter along the most wonderful shore of the North-West of France.
Between Le Havre and Dieppe you will see Etretat (photo below), Veulettes, Saint Valery, Veules les roses and many other little cities or villages.
IMAGE LINK: http://www.hostingpics.net …php?id=925917Walking1.jpg
But maybe this shore will not change your habits because, with the high white cliffs, it looks like the beautiful southern coast of England...
After Dieppe and more to North I know less.
Baie de la Somme, around Abbeville and Le Crotoy is a vast calm area where you can observe thousands and thousands of migratory birds.
I'm sorry to be so imprecise, but the question is vast and the area you plan to visit is very large.
Anyway, if you have a question about precise place, tell me, I'll answer at my best.